> 2023年8月1日 上午4:06，Paul E. McKenney <paulmck@xxxxxxxxxx> 写道：
>
> On Sat, Jul 29, 2023 at 11:17:31PM +0800, Alan Huang wrote:
>>
>>> 2023年7月25日 00:09，Paul E. McKenney <paulmck@xxxxxxxxxx> 写道：
>>>
>>> On Sat, Jul 22, 2023 at 09:02:46AM +0800, Alan Huang wrote:
>>>>
>>>>> 2023年7月22日 上午6:49，Paul E. McKenney <paulmck@xxxxxxxxxx> 写道：
>>>>>
>>>>> On Wed, Jul 19, 2023 at 12:08:33AM +0800, Alan Huang wrote:
>>>>>>
>>>>>>> 2023年7月18日 03:06，Paul E. McKenney <paulmck@xxxxxxxxxx> 写道：
>>>>>>>
>>>>>>> On Tue, Jul 18, 2023 at 01:53:10AM +0800, Alan Huang wrote:
>>>>>>>>> 2023年7月18日 00:02，Paul E. McKenney <paulmck@xxxxxxxxxx> 写道：
>>>>>>>>> On Sun, Jul 16, 2023 at 07:21:28PM +0800, Alan Huang wrote:
>>>>>>>>>>> 2023年7月16日 01:19，Paul E. McKenney <paulmck@xxxxxxxxxx> 写道：
>>>>>>>>>>> On Sat, Jul 15, 2023 at 08:50:23AM +0800, Alan Huang wrote:
>>>>>>>>>>>>> 2023年7月15日 07:23，Paul E. McKenney <paulmck@xxxxxxxxxx> 写道：
>>>>>>>>>>>>> On Tue, Jul 11, 2023 at 03:09:06PM +0000, Alan Huang wrote:
>>>>>>>>>>>>>> The objects are allocated with SLAB_TYPESAFE_BY_RCU, and there is
>>>>>>>>>>>>>> n->next = first within hlist_add_head_rcu() before rcu_assign_pointer(),
>>>>>>>>>>>>>> which modifies obj->obj_node.next. There may be readers holding the
>>>>>>>>>>>>>> reference of obj in lockless_lookup, and when updater modifies ->next,
>>>>>>>>>>>>>> readers can see the change immediately because ofSLAB_TYPESAFE_BY_RCU.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> There are two memory ordering required in the insertion algorithm,
>>>>>>>>>>>>>> we need to make sure obj->key is updated before obj->obj_node.next
>>>>>>>>>>>>>> and obj->refcnt, atomic_set_release is not enough to provide the
>>>>>>>>>>>>>> required memory barrier.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Signed-off-by: Alan Huang <mmpgouride@xxxxxxxxx>
>>>>>>>>>>>>>
>>>>>>>>>>>>> This is an interesting one!!!
>>>>>>>>>>>>>
>>>>>>>>>>>>> Now I am having a hard time believing that the smp_rmb() suffices.
>>>>>>>>>>>>>
>>>>>>>>>>>>>> ---
>>>>>>>>>>>>>> Changelog:
>>>>>>>>>>>>>> v1 -> v2: Use _ONCE to protect obj->key.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Documentation/RCU/rculist_nulls.rst | 21 +++++++++++++--------
>>>>>>>>>>>>>> 1 file changed, 13 insertions(+), 8 deletions(-)
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> diff --git a/Documentation/RCU/rculist_nulls.rst b/Documentation/RCU/rculist_nulls.rst
>>>>>>>>>>>>>> index 21e40fcc08de..2a9f5a63d334 100644
>>>>>>>>>>>>>> --- a/Documentation/RCU/rculist_nulls.rst
>>>>>>>>>>>>>> +++ b/Documentation/RCU/rculist_nulls.rst
>>>>>>>>>>>>>> @@ -47,7 +47,7 @@ objects, which is having below type.
>>>>>>>>>>>>>> * reuse these object before the RCU grace period, we
>>>>>>>>>>>>>> * must check key after getting the reference on object
>>>>>>>>>>>>>> */
>>>>>>>>>>>>>> - if (obj->key != key) { // not the object we expected
>>>>>>>>>>>>>> + if (READ_ONCE(obj->key) != key) { // not the object we expected
>>>>>>>>>>>>>> put_ref(obj);
>>>>>>>>>>>>>> rcu_read_unlock();
>>>>>>>>>>>>>> goto begin;
>>>>>>>>>>>>>> @@ -64,10 +64,10 @@ but a version with an additional memory barrier (smp_rmb())
>>>>>>>>>>>>>> {
>>>>>>>>>>>>>> struct hlist_node *node, *next;
>>>>>>>>>>>>>> for (pos = rcu_dereference((head)->first);
>>>>>>>>>>>>>> - pos && ({ next = pos->next; smp_rmb(); prefetch(next); 1; }) &&
>>>>>>>>>>>>>> + pos && ({ next = READ_ONCE(pos->next); smp_rmb(); prefetch(next); 1; }) &&
>>>>>>>>>>>>>
>>>>>>>>>>>>> Suppose that lockless_lookup() is delayed just before fetching pos->next,
>>>>>>>>>>>>> and that there were 17 more node to search in the list.
>>>>>>>>>>>>>
>>>>>>>>>>>>> Then consider the following sequence of events:
>>>>>>>>>>>>>
>>>>>>>>>>>>> o The updater deletes this same node and kmem_cache_free()s it.
>>>>>>>>>>>>>
>>>>>>>>>>>>> o Another updater kmem_cache_alloc()s that same memory and
>>>>>>>>>>>>> inserts it into an empty hash chain with a different key.
>>>>>>>>>>>>>
>>>>>>>>>>>>> o Then lockless_lookup() fetches pos->next and sees a NULL pointer,
>>>>>>>>>>>>> thus failing to search the remaining 17 nodes in the list,
>>>>>>>>>>>>> one of which had the desired key value.
>>>>>>>>>>>>>
>>>>>>>>>>>>> o The lookup algorithm resumes and sees the NULL return from
>>>>>>>>>>>>> lockless_lookup(), and ends up with a NULL obj.
>>>>>>>>>>>>>
>>>>>>>>>>>>> And this happens even with the strongest possible ordering
>>>>>>>>>>>>> everywhere.
>>>>>>>>>>>>>
>>>>>>>>>>>>> OK, yes, it is late on Friday. So what am I missing here?
>>>>>>>>>>>>
>>>>>>>>>>>> You missed nothing!
>>>>>>>>>>>>
>>>>>>>>>>>> The lockless_lockup should not be a function, but a macro like hlist_for_each_entry_rcu.
>>>>>>>>>>>
>>>>>>>>>>> How would you fix this using a macro?
>>>>>>>>>>
>>>>>>>>>> With additional detection code. A moved object (in another chain) will have a different slot.
>>>>>>>>>> (I have sent patch v3. )
>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>>> Independent of that, does hlist_add_head_rcu() need to replace its
>>>>>>>>>>>>> "n->next = first" with "WRITE_ONCE(n->next, first)"?
>>>>>>>>>>>>
>>>>>>>>>>>> I think users who want to use hlist with SLAB_TYPESAFE_BY_RCU should use rculist_nulls?
>>>>>>>>>>>
>>>>>>>>>>> I believe that you are correct. Would you like to propose a patch, or
>>>>>>>>>>> would you rather I put something together? My current thought is to
>>>>>>>>>>
>>>>>>>>>> Feel free to add.
>>>>>>>>>>
>>>>>>>>>> One thing I think would be useful is to tell readers where the ‘next' is.
>>>>>>>>>> The document mentions ’next’ many times, but it’s hard for me, as a reader, to realize that
>>>>>>>>>> the ‘next' is within hlist_add_head_rcu(). (I have no idea where to put the hint.)
>>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>>> keep the examples, but to show why the one with smp_rmb() is broken.
>>>>>>>>>>
>>>>>>>>>> I think the example needs to be fixed. :)
>>>>>>>>>
>>>>>>>>> Even better! I will take a look, but in the meantime, would you be
>>>>>>>>> interested in updating the wording to explain how the back-pointer works?
>>>>>>>>
>>>>>>>> Which document needs to be updated?
>>>>>>>> And is there anything that I can refer to? It’s the first time I have ever heard about it.
>>>>>>>
>>>>>>> Documentation/RCU/rculist_nulls.rst, the one that you are updating.
>>>>>>>
>>>>>>> There admittedly isn't a whole lot of commentary.
>>>>>>>
>>>>>>>>> (Looks similar to the is_a_nulls() pointer, but in each element instead of
>>>>>>>>> just at the end. One advantage is the ability to detect a move mid-list,
>>>>>>>>> though that is not a big deal in well-tuned hash tables, which tend to
>>>>>>>>> have short hash chains. The need to move elements to the front of the
>>>>>>>>> destination list remains, though in both cases only if it has been less
>>>>>>>>> than a grace period since the last move.)
>>>>>>>>
>>>>>>>> Looks like that I need to learn it first. :)
>>>>>>>
>>>>>>> Well, you wrote the code, so... ;-)
>>>>>>
>>>>>> If I understand correctly, it works only for 64-bit machines?
>>>>>>
>>>>>> And the number of slots of the hash table will be limited?
>>>>>
>>>>> You are asking about the is_a_nulls() value? If so, it works on both
>>>>> 32-bit and 64-bit machines. They each have enough bits for the nulls
>>>>> value to cover all possible two-byte objects in the full address space.
>>>>>
>>>>> If that wasn't what you were asking, please help me with your question.
>>>>
>>>> I’m asking the ‘back-pointer’, which
>>>>
>>>> "Looks similar to the is_a_nulls() pointer, but in each element instead of
>>>> just at the end”
>>>>
>>>> If I understand correctly, we need to store the nulls in the upper unused bits.
>>>>
>>>> And we only have several bits unused. One advantage I can think of is that it
>>>> will improve the performance once we have something like the Intel Upper Address Ignore (UAI),
>>>> which also works only for 64-bit machine.
>>>
>>> Ah, I was using the word "similar" very loosely. Not "implemented in
>>> a manner similar to is_a_nulls(), but rather "serves roughly the same
>>> function as is_a_nulls()".
>>
>> I got it.
>>
>>>
>>> You should be able to just have the pointer to the bucket in each
>>> element, and just compare for each element. No need for extra bits,
>>
>> It might have the same functionality if we put the nulls value in each element.
>
> There are some strange things that are easier to do with a nulls value
> than with an explicit pointer, but yes.
>
> (For example, if for some odd reason you wanted to group the hash buckets
> into classes within which moving a reader to another bucket was OK.
> Why would you want to do this? I have no idea!)
Interesting example.
I’ll try to update the wording to explain how the back-pointer works.
By the way, would you like a separate patch or a patch V4 containing the content of
V3 of "docs/RCU: Bring back smp_wmb()” and the back-pointer?
>
> Thanx, Paul
>
>>> though UAI, like its pre-existing ARM counterpart, might also help catch
>>> use-after-free bugs when resizing the hash table.
>>>
>>> Thanx, Paul
>>>
>>>>>>>>>>>> I didn’t find a case using hlist with SLAB_TYPESAFE_BY_RCU, but I did find a case using list
>>>>>>>>>>>> with SLAB_TYPESAFE_BY_RCU in drivers/gpu/drm/i915, the driver also doesn’t use _ONCE
>>>>>>>>>>>> on the fields of the objects allocated with SLAB_TYPESAFE_BY_RCU.
>>>>>>>>>>>
>>>>>>>>>>> Feel free to send them a patch, though I cannot speak for their
>>>>>>>>>>> reception of it.
>>>>>>>>>>>
>>>>>>>>>>> Thanx, Paul
>>>>>>>>>>>
>>>>>>>>>>>>> Thanx, Paul
>>>>>>>>>>>>>
>>>>>>>>>>>>>> ({ obj = hlist_entry(pos, typeof(*obj), obj_node); 1; });
>>>>>>>>>>>>>> pos = rcu_dereference(next))
>>>>>>>>>>>>>> - if (obj->key == key)
>>>>>>>>>>>>>> + if (READ_ONCE(obj->key) == key)
>>>>>>>>>>>>>> return obj;
>>>>>>>>>>>>>> return NULL;
>>>>>>>>>>>>>> }
>>>>>>>>>>>>>> @@ -111,8 +111,13 @@ detect the fact that it missed following items in original chain.
>>>>>>>>>>>>>> */
>>>>>>>>>>>>>> obj = kmem_cache_alloc(...);
>>>>>>>>>>>>>> lock_chain(); // typically a spin_lock()
>>>>>>>>>>>>>> - obj->key = key;
>>>>>>>>>>>>>> - atomic_set_release(&obj->refcnt, 1); // key before refcnt
>>>>>>>>>>>>>> + WRITE_ONCE(obj->key, key);
>>>>>>>>>>>>>> + /*
>>>>>>>>>>>>>> + * We need to make sure obj->key is updated before obj->obj_node.next
>>>>>>>>>>>>>> + * and obj->refcnt.
>>>>>>>>>>>>>> + */
>>>>>>>>>>>>>> + smp_wmb();
>>>>>>>>>>>>>> + atomic_set(&obj->refcnt, 1);
>>>>>>>>>>>>>> hlist_add_head_rcu(&obj->obj_node, list);
>>>>>>>>>>>>>> unlock_chain(); // typically a spin_unlock()
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> @@ -165,12 +170,12 @@ Note that using hlist_nulls means the type of 'obj_node' field of
>>>>>>>>>>>>>> begin:
>>>>>>>>>>>>>> rcu_read_lock();
>>>>>>>>>>>>>> hlist_nulls_for_each_entry_rcu(obj, node, head, obj_node) {
>>>>>>>>>>>>>> - if (obj->key == key) {
>>>>>>>>>>>>>> + if (READ_ONCE(obj->key) == key) {
>>>>>>>>>>>>>> if (!try_get_ref(obj)) { // might fail for free objects
>>>>>>>>>>>>>> rcu_read_unlock();
>>>>>>>>>>>>>> goto begin;
>>>>>>>>>>>>>> }
>>>>>>>>>>>>>> - if (obj->key != key) { // not the object we expected
>>>>>>>>>>>>>> + if (READ_ONCE(obj->key) != key) { // not the object we expected
>>>>>>>>>>>>>> put_ref(obj);
>>>>>>>>>>>>>> rcu_read_unlock();
>>>>>>>>>>>>>> goto begin;
>>>>>>>>>>>>>> @@ -206,7 +211,7 @@ hlist_add_head_rcu().
>>>>>>>>>>>>>> */
>>>>>>>>>>>>>> obj = kmem_cache_alloc(cachep);
>>>>>>>>>>>>>> lock_chain(); // typically a spin_lock()
>>>>>>>>>>>>>> - obj->key = key;
>>>>>>>>>>>>>> + WRITE_ONCE(obj->key, key);
>>>>>>>>>>>>>> atomic_set_release(&obj->refcnt, 1); // key before refcnt
>>>>>>>>>>>>>> /*
>>>>>>>>>>>>>> * insert obj in RCU way (readers might be traversing chain)
>>>>>>>>>>>>>> --
>>>>>>>>>>>>>> 2.34.1
